Mechanisms Flashcards
What condition is required for free radical substitution?
UV light
What is a free radical?
A reactive species with an unpaired electron
What happens during free radical substitution?
In the presence of UV light a halogen substitutes for a hydrogen atom in an alkane/halogenoalkane to produce a halogenolakane and a hydrogen halide.
What is heterolytic fission?
An endothermic process. Energy is added and the covalent bond breaks unevenly, both electrons going to the same atom
What is homolytic fission?
An endothermic process. Energy is added and the covalent bond breaks evenly, one electron going to each atom producing free radicals.
What are the steps in free radical substitution?
Initiation Propagation Termination
Describe initiation in free radical substitution
A halogen molecule splits homolytically in the presence of UV light. Molecule produces a free radical.
Describe propagation in free radical substitution
Molecule and a free radical react to produce a molecule and a free radical.
A) halogen free radical removes hydrogen from alkane.
B)Alkane free radical reacts with halogen
This is a chain reaction process.
Describe termination in free radical substitution.
Free radicals react to form a molecule.
End of the reaction
How can an excess of reactants be used to influence the products of free radical substitution?
Excess of alkane = one substitution
Excess of halogen = many substitutions.
What is a nucleophile?
A lone pair donator.
Generally a negative ion or molecule with delta negative charge.
What happens during nucleophilic substitution?
A nucleophlie replaces a halogen on a molecule.
Nucleophile is attracted to delta positive carbon caused by polar bond with an electronegative halogen.
Give the possible nucleophiles in nucleophilic substitution.
:OH- hydroxide ion
:CN- cyanide ion
:NH3 ammonia
Give the conditions for a hydroxide ion to act as a nucleophile in a nucleophilic substitution.
aqueous, warm, dilute
Give the conditions for a cyanide ion to act as a nucleophile in nucleophilic substitution.
ethanolic, warm
Give the conditions for ammonia to act as a nucleophile in nucleophilic substitution.
Excess concentrated ammonia dissolved in ethanol
(in a seeled container at constant pressure)
Name the organic product for nucleophilic substitution with an hydroxide ion
Alcohol
eg methanol CH3OH
Name the organic product for nucleophilic substitution with cyanide ions
Nitrile
e.g ethannitrile CH3CN
Name the organic product of nucleophilic substitution with a ammonia nucleophile
amine
e.g ethylamine CH3CH2NH2
Draw the mechanism for nucleophilic substitution with a hydroxide ion.
Draw the mechanism for nucleophilic substitution with a cyanide ion nucleophile
Draw the mechanism for nucleophilic substitution with an ammonia nucleophile.
What a substitution reaction?
A reaction when an atom or group replaces another atom or group on the molecule
what is an addition reaction?
A reaction where a group/atom is added to a unsaturated molecule to produce a saturated molecule.
what is an electrophile?
A lone elctron pair acceptor
Usually cations or delta positive
What happens during an elimination reaction?
A hydrogen and halogen are removed from the saturated alkane to form an unstaurated alkene, water and a salt.
What is a base?
A proton (H+) acceptor
what is an acid?
A proton (H+) donor
What is an alkali?
A soluble base.
Produces OH- ions in water.
What are the conditions for an elimaination reaction?
hot, ethanolic, concentrated potassium hydroxide
What type of halogenoalkane favours elimination/nucleophilic substitution?
Primary favours nucleophilic substitution
Tertiary favours elimination
what does the hydroxide ion act as in a elimination reaction?
A base
What hapens during electrohilic addition?
The carbon doudle bond opens and an atoms/groups are added to each carbon atom. This forms a saturated alkane from and unsaturated alkene.
Name the intermediate of electrophilic addition
A carbocation
What is the major/minor product?
Major - the product in highest abundance
minor - the product in lowest abundance
Why are alkenes suseptible to electrophilic addition?
The carbon-carbon double bond occurs in the overlap of two p orbitals. This creates an elctron rich π bond which attracts the positively charged electrophile.
What is the positive inductile effect?
The ability of a alkyl group to repel electrons away from itself.
How do you determin the major product from electrophilic addition?
The more stable carbocation will form the major product.
The more stable carbocation will by have the reaction occuring at a carbon attached to more alkyl groups (increased positive inductile effect). These will push more electrons towards the carbocation
Give the conditions for electrophilic addition with bromine
aqueous
Give the conditions for electrophilic addition with sulfuric acid
Concentrated sulfuric acid (H2SO4), cold (room temperature)
Give the product for electrophilic addition with sulphuric acid
an alkyl hydrogen sulphate
What makes the product of electrophilic addition with sulphuric acid useful?
The product is an alkyl hydrogen sulphate. Adding water will give an alcohol (and sulphuric acid)
Give the conditions for an elctrophilic addition reaction with hydrogen bromide
cold (room temperature)
Give the reaction conditions for elctrophilic addition with water with a strong acid.
strong acid must be dissolved in the water.
what is meant by a strong acid?
An acid that fully dissociates in water, gives a large amount of H+ ions in solution.
what is interesting about electrophilic addition with water and a strong acid.
H+ ions from strong acid acid as electrophile
Produces an intermediate carbocation. This reacts with the lone pair of an oxygen on a water molecule.
A second intermediate is produced with a cation oxygen. This attracts the electrons in the oxygen hydrogen bond so hydrogen ions leave.
Hydrogen ions act as a catalyst.
draw the reaction mechanism for an elimination reaction
draw the mechanism for a bromine electrophilic addition reaction
draw the mechanism for an hydrogen bromide electrophilic addition reaction
draw the mechanism for elctrophiic addition in water with a strong acid
draw the reaction mechanism for electrophilic addition with sulphuric acid.
Draw the structure of sulphuric acid
Give all the possible electrophiles in electrophilic addition.
bromine (istantaneous dipole forms delta positive)
Hydrogen brominde (hydrogen delta positive)
Sulphuric acid (hydrogen delta positive)
Water with strong acid (hydrogen ion)
Name the organic product from an electrophilic addition reaction with sulphuric acid
alkyl hydrogen sulphate
name the organic product from an electrophilic addition with water and a strong acid
an alcohol
